1. Field of the Invention
This invention relates an apparatus and a method for localizing a sound image, more specifically the simplification of its structure and the processes.
2. Description of the Prior Art
An apparatus for localizing a sound image disclosed in Japanese Laid-open publication No. Hei 8-265899 (265899/1996) is shown in FIG. 9. The apparatus is used to make a listener 2 to feel that sound image reproduced by speakers XL and XR (hereinafter referred to as virtual speakers) is virtually localized at rear sides to the listener 2. By utilizing the apparatus, the listener 2 is able to feel like that he/she is surrounded by the sound reproduced with the speakers 4L and 4R as well as surrounded by the sound reproduced with the virtual speakers XL and XR even when only the speakers 4L and 4R are actually arranged.
In the apparatus shown in FIG. 9, a total of four filters 6a, 6b, 6c and 6d are used to realize the sound image localization. Transfer functions H11, H12, H13 and H14 of respective filters are shown as following equations: EQU H11=(h.sub.RR h.sub.L'L -h.sub.RL h.sub.L'R)/(h.sub.LL h.sub.RR -h.sub.LR h.sub.RL) EQU H12=(h.sub.LL h.sub.L'R -h.sub.LR h.sub.L'L)/(h.sub.LL h.sub.RR -h.sub.LR h.sub.RL) EQU H21=(h.sub.RR h.sub.R'L -h.sub.RL h.sub.R'R)/(h.sub.LL h.sub.RR -h.sub.LR h.sub.RL) EQU H22=(h.sub.LL h.sub.R'R -h.sub.LR h.sub.R'L)/(h.sub.LL h.sub.RR -h.sub.LR h.sub.RL)
Wherein h.sub.RR is a transfer function from the speaker 4R to the right ear 2R of the listener 2, h.sub.RL is a transfer function from the speaker 4R to the left ear 2L of the listener 2, h.sub.LL is a transfer function from the speaker 4L to the left ear 2L of the listener 2, and h.sub.LR is a transfer function from the speaker 4L to the right ear 2R of the listener 2.
Incidentally, equations h.sub.LL =h.sub.RR, h.sub.LR =h.sub.RL, h.sub.L'L =h.sub.R'R, h.sub.L'R =h.sub.R'L are satisfied in the equations stated above when the speakers 4R, 4L and the speakers XR, XL are arranged symmetric with respect to a central axis 8 through the listener 2. As a result, equations H11=H22, and H12=H21 can be derived, so that the apparatus can be realized by utilizing a total of two filers as shown in FIG. 10. Here, transfer functions H.sub.SUM, H.sub.DIF can be defined by the following equations: EQU H.sub.SUM =(h.sub.a' +h.sub.b')/(h.sub.a +h.sub.b) EQU H.sub.DIF =(h.sub.a' -h.sub.b)/(h.sub.a -h.sub.b)
wherein equations h.sub.a =h.sub.LL =h.sub.RR, h.sub.b =h.sub.LR =h.sub.RL, h.sub.a '=h.sub.L'L =h.sub.R'R and h.sub.b '=h.sub.L'R =h.sub.R'L are satisfied.
Thus, the sound images can be localized at positions of the speakers arranged virtually with a simple structure when the actual speakers are symmetrically arranged.
Although, a sound effect so called "surround-effect" can be achieved by using a total of two speakers in the conventional technology, not much attention is paid to widen a width of frontal sound field (hereinafter referred to as frontal width) defined between the speakers arranged in a front side. Therefore, it is not possible to enjoy the "surround-effect" at sufficient level because of insufficient frontal width in an electric appliance such as a television set having a limited width for installing speakers therein.
Further, a technology to localize virtual speakers to outward of the front speakers is disclosed in Japanese Laid-open publication No. SHO 52-116202 (116202/1977). Although, the frontal width can be widen by applying the technology to both signals for left and right channels, additional circuits respectively carrying out localization of both the channels are required for widening the frontal width in addition to a circuit to perform processings of surround channel signals.
Still further, a technology achieving the "surround-effect" by using processings for localizing a sound image with respect to a surround channel signal is also disclosed in both Japanese Laid-open publications No. Hei 7-95697 (95697/1995) and No. Hei 7-212898 (212898/1995). However, the technologies disclosed therein do not allow to widen the frontal width.